minor #12989 [Messenger] Note about serializer (Nyholm)

This PR was submitted for the master branch but it was merged into the 4.4 branch instead (closes #12989).

Discussion
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[Messenger] Note about serializer

The issue is that I've multiple times heard people saying:

> I like Messenger component, but it does not work if you send messages from one app to another.

First I wanted to solve this issue by adding a new page just saying "YES YOU CAN".

But for now, I settle with a small note and a link to Symfony Cast explaining this.

There is also a great over engineered package that I believe everybody should use... but I dont feel link symfony docs is a place to promote a package like this, or is it? https://github.com/Happyr/message-serializer

I think this will fix symfony/symfony#35439

Commits
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09d4c7a [Messenger] Note about serializer

Author: javiereguiluz

messenger.rst

@@ -1023,6 +1023,13 @@ If you *do* choose to use the Symfony serializer, you can control the context
 on a case-by-case basis via the :class:`Symfony\\Component\\Messenger\\Stamp\\SerializerStamp`
 (see `Envelopes & Stamps`_).
 
+.. tip::
+
+    When sending/receiving messages to/from another application, it could be a good
+    idea to have more control over the serialization process. Using a custom serializer
+    provides that control. See `Symfony Casts' message serializer tutorial`_ for
+    details.
+
 Customizing Handlers
 --------------------
 
@@ -1514,3 +1521,4 @@ Learn more
 .. _`Enqueue's transport`: https://github.com/sroze/messenger-enqueue-transport
 .. _`streams`: https://redis.io/topics/streams-intro
 .. _`Supervisor docs`: http://supervisord.org/
+.. _`Symfony Casts' message serializer tutorial`: https://symfonycasts.com/screencast/messenger/transport-serializer